Conventional engine-driven forklift trucks have been equipped with oil pressure switches to monitor the amount of the engine oil. When the truck is in operation, if the amount of the engine oil decreases below a certain level, the oil pressure switch is turned on. This causes a warning lamp to light up, informing the human operator of lack of the oil.
However, the operator can continue the operation even after the warning lamp has lit up. Accordingly, the operator might continue to run the vehicle while neglecting the warning. In this case, the engine may be burned out or other trouble may take place.